Flow battery

ABSTRACT

A flow battery includes a first liquid containing a first electrode mediator dissolved therein, a first electrode immersed in the first liquid, a first active material immersed in the first liquid, and a first circulation mechanism that circulates the first liquid between the first electrode and the first active material, wherein the first electrode mediator includes a tetrathiafulvalene derivative, and the tetrathiafulvalene derivative has a chain-forming substituent at positions 4,4′ and 5,5′ of a tetrathiafulvalene skeleton thereof.

BACKGROUND 1. Technical Field

The present disclosure relates to a flow battery.

2. Description of the Related Art

Japanese Unexamined Patent Application Publication (Translation of PCTApplication) No. 2014-524124 discloses a redox flow battery system thatincludes an energy storage containing a redox mediator.

SUMMARY

There is a demand for a flow battery with a high discharge potential.

In one general aspect, the techniques disclosed here feature a flowbattery that includes a first liquid containing a first electrodemediator dissolved therein, a first electrode immersed in the firstliquid, a first active material immersed in the first liquid, and afirst circulation mechanism that circulates the first liquid between thefirst electrode and the first active material, wherein the firstelectrode mediator includes a tetrathiafulvalene derivative, and thetetrathiafulvalene derivative has a chain-forming substituent atpositions 4,4′ and 5,5′ of a tetrathiafulvalene skeleton thereof.

The flow battery can have a higher discharge potential.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a flow battery according to a firstembodiment;

FIG. 2 is a table that lists the measured electric potentials oftetrathiafulvalene derivatives usable as a first electrode mediator;

FIG. 3 is a schematic view of a flow battery according to a secondembodiment;

FIG. 4 is a block diagram of a flow battery according to a thirdembodiment;

FIG. 5 is a table that lists the electric potentials of condensedaromatic compounds usable as a charge mediator;

FIG. 6 is a table that lists the electric potentials of condensedaromatic compounds usable as a discharge mediator;

FIG. 7 is a table that lists the estimated energy density of the flowbattery according to the third embodiment; and

FIG. 8 is a schematic view of a flow battery according to a fourthembodiment.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described below withreference to the accompanying drawings.

First Embodiment

FIG. 1 is a block diagram of a flow battery 1000 according to a firstembodiment.

The flow battery 1000 according to the first embodiment includes a firstliquid 110, a first electrode 210, a first active material 310, and afirst circulation mechanism 510.

The first liquid 110 contains a first electrode mediator 111 dissolvedtherein.

The first electrode 210 is immersed in the first liquid 110.

The first active material 310 is immersed in the first liquid 110.

The first circulation mechanism 510 circulates the first liquid 110between the first electrode 210 and the first active material 310.

The first electrode mediator 111 includes a tetrathiafulvalenederivative.

The tetrathiafulvalene derivative has a chain-forming substituent atpositions 4,4′ and 5,5′ of the tetrathiafulvalene skeleton thereof.

Such a structure can provide a flow battery with a high dischargepotential, high energy density, and long cycle life.

In such a structure, the chain-forming substituent (electron-withdrawingsubstituent) at positions 4,4′ and 5,5′ of the tetrathiafulvaleneskeleton of the tetrathiafulvalene derivative can make it difficult forthe tetrathiafulvalene derivative to release an electron. When comparedwith tetrathiafulvalene derivatives without the substituent (forexample, approximately 3.2 V vs. Li/Li⁺), this can increase the chargepotential and discharge potential. Thus, even when an active materialwith a high equilibrium potential (for example, approximately 3.5 V vs.Li/Li⁺) is used as the first active material 310, the mediator can havea discharge potential closer to the equilibrium potential of the firstactive material 310. Thus, the flow battery can have a higher dischargepotential.

Furthermore, such a structure can provide a flow battery in which anactive material is not circulated. Thus, a high-capacity active materialpowder can be used as the first active material 310, for example, in acharge-discharge reaction. Thus, high energy density and capacity can beachieved.

Such a structure can circulate only the first liquid 110 containing thefirst electrode mediator 111 dissolved therein without circulating anactive material powder. This can reduce the occurrence of clogging of apipe with the active material powder. Thus, the flow battery can have along cycle life.

The tetrathiafulvalene derivative in the flow battery 1000 according tothe first embodiment may be represented by the following general formula(1).

In the general formula (1), X denotes an oxygen atom, a sulfur atom, anitrogen atom, a selenium atom, or a tellurium atom, R₁, R₂, R₃, andindependently denote at least one selected from the group consisting ofchain saturated hydrocarbons, chain unsaturated hydrocarbons, cyclicsaturated hydrocarbons, cyclic unsaturated hydrocarbons, a phenyl group,a hydrogen atom, a hydroxy group, a cyano group, an amino group, a nitrogroup, and a nitroso group.

Such a structure can provide a flow battery with a higher dischargepotential.

The hydrocarbons may have at least one selected from an oxygen atom, anitrogen atom, a sulfur atom, and a silicon atom.

In the flow battery 1000 according to the first embodiment, atetrathiafulvalene derivative represented by the general formula (1) mayhave at least one electron-withdrawing group selected from the groupconsisting of a sulfur atom, a nitrogen atom, and an oxygen atom atposition X.

Such a structure can provide a flow battery with a higher dischargepotential due to the presence of an electron-withdrawing atom. Inparticular, with a sulfur atom at position X, the flow battery can havea higher discharge potential.

In the flow battery 1000 according to the first embodiment, atetrathiafulvalene derivative represented by the general formula (1) mayhave a linear substituent at positions R₁, R₂, R₃, and

Such a structure can provide a flow battery with a higher dischargepotential.

In the flow battery 1000 according to the first embodiment, thesubstituent at positions R₁, R₂, R₃, and R₄ of a tetrathiafulvalenederivative represented by the general formula (1) may be C_(n)H_(2n+1)(n is an integer of 1≦n≦4).

Such a structure can provide a flow battery with a higher dischargepotential. Furthermore, low solubility of tetrathiafulvalene derivativeswith n≦5 (particularly when the substituent is a chain saturatedhydrocarbon alone) due to increased melting points can be avoided. Morespecifically, the use of —CH₃, —CH₂CH₃, —CH₂CH₂CH₃, or —CH₂CH₂CH₂CH₃ asa substituent at positions R₁, R₂, R₃, and R₄ can suppress the decreasein the solubility of the tetrathiafulvalene derivative in the firstliquid 110.

In the flow battery 1000 according to the first embodiment, thesubstituent at positions R₁, R₂, R₃, and R₄ may be a branchedsubstituent.

Such a structure can provide a flow battery with a higher dischargepotential.

In the flow battery 1000 according to the first embodiment, thesubstituent at positions R₁, R₂, R₃, and R₄ of a tetrathiafulvalenederivative represented by the general formula (1) may have at least oneelement (non-metallic element) selected from the group consisting ofboron, nitrogen, oxygen, fluorine, silicon, phosphorus, sulfur,chlorine, bromine, and iodine.

Such a structure can provide a flow battery with a higher dischargepotential.

In the flow battery 1000 according to the first embodiment, thetetrathiafulvalene derivative may be at least one selected from thegroup consisting of tetrakis(dimethylthio)tetrathiafulvalene andtetrakis(diethylthio)tetrathiafulvalene.

Such a structure can provide a flow battery with a higher dischargepotential.

FIG. 2 is a table that lists the measured electric potentials oftetrathiafulvalene derivatives usable as a first electrode mediator 111.

An electrolyte solution of 1 M electrolyte (LiBF₄) in a propylenecarbonate solvent was prepared. A compound listed in FIG. 2 wasdissolved in the electrolyte solution at a concentration of 2 mM toprepare an electrolyte solution of the compound listed in FIG. 2. Anelectrometric cell for each compound listed in FIG. 2 was fabricatedfrom the electrolyte solution, a counter electrode (1×1 cm Pt foil), aworking electrode (glassy carbon electrode for electrochemicalmeasurement (φ6 mm)), and a reference electrode (silver wire (Ag/Ag⁺)).The electrometric cell was used to measure the charge-dischargepotential of each compound listed in FIG. 2 by cyclic voltammetry (CV),FIG. 2 lists the measured charge-discharge potentials based on lithiummetal (V vs. Li/Li⁺).

Tetrathiafulvalene and an oxidized derivative thereof have aromaticityand undergo two-stage oxidation-reduction reactions. Thus,tetrathiafulvalene or a derivative thereof can be used alone as acharge-discharge mediator. The equilibrium potential of the first-stageoxidation-reduction reaction (first oxidation-reduction potential: E1 (Vvs. Li/Li⁺)) corresponds to the electric potential of the dischargemediator. The equilibrium potential of the second-stageoxidation-reduction reaction (second oxidation-reduction potential: E2(V vs. Li/Li⁺)) corresponds to the electric potential of the chargemediator.

Among the compounds listed in FIG. 2, the tetrathiafulvalene derivativesaccording to the first embodiment (tetrathiafulvalene derivatives with achain-forming substituent) tetrakis(dimethylthio)tetrathiafulvalene andtetrakis(diethylthio)tetrathiafulvalene have a higher firstoxidation-reduction potential E1 (discharge potential) thantetrathiafulvalenes with no chain-forming substituent.

The discharge potential of a flow battery depends on the electricpotential of a positive-electrode discharge mediator. Thus, thetetrathiafulvalene derivatives according to the first embodiment(tetrathiafulvalene derivatives with a chain-forming substituent) with ahigher discharge potential than tetrathiafulvalenes with nochain-forming substituent (with a discharge potential of approximately3.2 V) can provide a flow battery with a higher discharge potential.

In the flow battery 1000 according to the first embodiment, the firstelectrode mediator 111 may contain only one tetrathiafulvalenederivative that satisfies the conditions of the tetrathiafulvalenederivatives according to the first embodiment.

Alternatively, in the flow battery 1000 according to the firstembodiment, the first electrode mediator 111 may contain two or moretetrathiafulvalene derivatives that satisfy the conditions of thetetrathiafulvalene derivatives according to the first embodiment.

As described above, a tetrathiafulvalene derivative according to thefirst embodiment has a first oxidation-reduction potential E1 and asecond oxidation-reduction potential E2.

The equilibrium potential of the first active material 310 (V vs.Li/Li⁺) may be higher than the first oxidation-reduction potential E1and lower than the second oxidation-reduction potential E2.

In such a structure, the use of an active material with an equilibriumpotential higher than the first oxidation-reduction potential E1 (withan electric potential higher than the first oxidation-reductionpotential E1) as the first active material 310 allows atetrathiafulvalene derivative according to the first embodiment tofunction as a discharge mediator. The use of an active material with anequilibrium potential lower than the second oxidation-reductionpotential E2 (with an electric potential lower than the secondoxidation-reduction potential E2) as the first active material 310allows a tetrathiafulvalene derivative according to the first embodimentto function as a charge mediator.

In the flow battery 1000 according to the first embodiment, the firstactive material 310 may be a metal oxide represented by Li_(x)M_(y)O₂. Mdenotes at least one selected from the group consisting of Ni, Mn, andCo. x and y may be any number. The metal oxide has an equilibriumpotential in the range of 3.2 to 3.8 V.

In the flow battery 1000 according to the first embodiment, the firstactive material 310 may be at least one selected from the groupconsisting of LiFePO₄, LiMnO₂, LiMn₂O₄, and LiCoO₂.

LiFePO₄ has an equilibrium potential of 3.5 V vs. Li/Li⁺. Thus, amediator-type positive electrode with a LiFePO₄ active material can beformed by using a compound with a discharge potential higher than theequilibrium potential of LiFePO₄ and with a charge potential lower thanthe equilibrium potential of LiFePO₄ as the first electrode mediator 111(a charge-discharge mediator). In this case, a smaller potentialdifference between the equilibrium potential of LiFePO₄ and thecharge-discharge potential of the first electrode mediator 111 resultsin higher charge-discharge energy efficiency. For example, a dischargepotential of the first electrode mediator 111 lower than the equilibriumpotential of LiFePO₄ and closer to the equilibrium potential of LiFePO₄results in a higher discharge potential of the flow battery.

Thus, if LiFePO₄ is used as the first active material 310, the use oftetrakis(dimethylthio)tetrathiafulvalene andtetrakis(diethylthio)tetrathiafulvalene as the first electrode mediator111 can increase the discharge potential of the flow battery. In thiscase, the discharge potential can be increased by approximately 0.1 Vcompared with using tetrathiafulvalene with no chain-forming substituent(with a discharge potential of approximately 3.2 V) as the firstelectrode mediator 111.

The first active material 310 may be a solid active material (forexample, a powdered active material). If the first active material 310is stored as an unprocessed powder in a tank, this can simplify theproduction and reduce production costs.

Alternatively, the first active material 310 may be a pelleted activematerial (for example, a powder is pelleted) If the first activematerial 310 is stored as pellets in a tank, this can simplify theproduction and reduce production costs.

The first active material 310 may be pelleted with a generally knownbinder (for example, poly(vinylidene difluoride), polypropylene,polyethylene, or polyimide).

The first active material 310 may be insoluble in the first liquid 110.Thus, there is provided a flow battery in which the first liquid 110 andthe first electrode mediator 111 circulate, but the first activematerial 310 does not circulate.

In the flow battery 1000 according to the first embodiment, the firstliquid 110 may be at least one selected from the group consisting ofpropylene carbonate (PC), ethylene carbonate (EC), γ-butyrolactone,dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethylcarbonate (DEC).

In the flow battery 1000 according to the first embodiment, the firstliquid 110 may be an electrolyte solution containing an electrolyte in asolvent, which is the material described above usable as the firstliquid 110. The electrolyte (salt) may be at least one selected from thegroup consisting of LiBF₄, LiPF₆, and LiN(CF₃SO₂)₂. The solvent may havea high dielectric constant, low reactivity with Li ions, and a potentialwindow up to approximately 4 V.

In the flow battery 1000 according to the first embodiment, the firstelectrode 210 may be a positive electrode, and a second electrode 220may be a negative electrode.

If the second electrode 220 has a relatively high electric potential,the first electrode 210 may function as a negative electrode.

Thus, the first electrode 210 may be a negative electrode, and thesecond electrode 220 may be a positive electrode.

In the flow battery 1000 according to the first embodiment, for example,when the first liquid 110 comes into contact with the first electrode210, the first electrode mediator 111 is oxidized or reduced on thefirst electrode 210.

The first electrode 210 may have a surface acting as a reaction fieldfor the first electrode mediator 111.

In this case, the material of the first electrode 210 may be stable inthe first liquid 110 (for example, a material insoluble in the firstliquid 110). The material of the first electrode 210 may also beresistant to an electrode reaction, that is, an electrochemicalreaction. For example, the first electrode 210 may be formed of a metal(stainless steel, iron, copper, or nickel) or carbon.

The first electrode 210 may have a structure with an increased surfacearea (for example, a mesh, nonwoven fabric, surface roughened plate, orsintered porous body). Thus, the first electrode 210 may have a largespecific surface area. This can promote an oxidation or reductionreaction of the first electrode mediator 111.

The second electrode 220 may include a current collector and an activematerial on the current collector. Thus, for example, a high-capacityactive material may be used. An active material of the second electrode220 may be a compound that reversibly adsorbs and desorbs lithium ions.

The second electrode 220 may be lithium metal. The second electrode 220made of lithium metal can easily control dissolution and precipitationas a metal positive electrode and achieve high capacity.

The flow battery 1000 according to the first embodiment may furtherinclude a separating unit 400.

The separating unit 400 separates the first electrode 210 and the firstliquid 110 from the second electrode 220.

The separating unit 400 may be a microporous film (porous body) for usein known secondary batteries.

Alternatively, the separating unit 400 may be a porous film, such asglass paper, which is a nonwoven fabric with glass fibers woven in.

Alternatively, the separating unit 400 may be a (lithium-)ion-conductingdiaphragm. For example, the separating unit 400 may be an ion-exchangeresin membrane (for example, a cation-exchange membrane oranion-exchange membrane) or a solid electrolyte membrane.

The first circulation mechanism 510 circulates the first liquid 110between the first electrode 210 and the first active material 310.

Such a structure can circulate the first electrode mediator 111 togetherwith the first liquid 110 between the first electrode 210 and the firstactive material 310. This can efficiently promote an oxidation reactionand a reduction reaction between materials.

The first circulation mechanism 510 may include a pipe, a tank, a pump,and a valve, for example.

A specific example of the first circulation mechanism 510 may be astructure described later in a second embodiment.

<Charge-Discharge Process>

The charge-discharge process of the flow battery 1000 according to thefirst embodiment will be described below.

The charge-discharge process is specifically described with thefollowing operation example.

In the operation example, the first electrode 210 is a positiveelectrode made of carbon black.

In the operation example, the first liquid 110 is an ether solutioncontaining the first electrode mediator 111 dissolved therein.

In the operation example, the first electrode mediator 111 is atetrathiafulvalene derivative according to the first embodiment(hereinafter referred to as “TTF”).

In the operation example, the first active material 310 is lithium ironphosphate (LiFePO₄).

In the operation example, the second electrode 220 is a negativeelectrode made of lithium metal.

[Charging Process]

First, a charge reaction will be described below.

A voltage is applied between the first electrode 210 and the secondelectrode 220 when charging.

Reaction on Negative Electrode

Upon application of a voltage, electrons are supplied to the negativeelectrode, that is, the second electrode 220 from the outside of theflow battery. A reduction reaction occurs on the negative electrode,that is, the second electrode 220. Thus, the negative electrode ischarged.

For example, in the operation example, the following reaction occurs.

Li⁺ +e ⁻→Li

Reaction on Positive Electrode

Upon application of a voltage, an oxidation reaction of the firstelectrode mediator 111 occurs on the positive electrode, that is, thefirst electrode 210. Thus, the first electrode mediator 111 is oxidizedon the surface of the first electrode 210. Thus, electrons are releasedfrom the first electrode 210 to the outside of the flow battery.

For example, in the operation example, the following reaction occurs.

TTF→TTF⁺ +e ⁻

TTF⁺→TTF²⁺ +e ⁻

The first circulation mechanism 510 transfers (supplies) the firstelectrode mediator 111 oxidized on the first electrode 210 to the firstactive material 310.

The first electrode mediator 111 oxidized on the first electrode 210 isreduced by the first active material 310. In other words, the firstactive material 310 is oxidized by the first electrode mediator 111.Thus, the first active material 310 desorbs lithium.

For example, in the operation example, the following reaction occurs.

LiFePO₄+TTF²⁺→FePO₄+Li⁺+TTF⁺

The first circulation mechanism 510 transfers (supplies) the firstelectrode mediator 111 reduced by the first active material 310 to thefirst electrode 210.

Thus, the first electrode mediator 111 is oxidized on the surface of thefirst electrode 210.

For example, in the operation example, the following reaction occurs.

TTF⁺→TTF²⁺ +e ⁻

Part of lithium ions (Li⁺) thus desorbed may move to the secondelectrode 220 through the separating unit 400.

Thus, the first electrode mediator 111 is unchanged in the wholereaction including circulation.

Meanwhile, the first active material 310 separated from the firstelectrode 210 is charged.

Thus, TTF⁺ acts as a charge mediator on the first electrode 210 (a firstelectrode side charge mediator).

In the fully charged state, the first liquid 110 contains TTF²⁺, and thefirst active material 310 is FePO₄. The charge potential depends on theoxidation potential with respect to the direction toward TTF²⁺.

The charge reaction can continue until the first active material 310 orthe second electrode 220 reaches the fully charged state.

[Discharge Process]

A discharge reaction starting from the fully charged state will bedescribed below.

In the fully charged state, the first active material 310 and the secondelectrode 220 are in the charged state.

During the discharge reaction, electric power generated between thefirst electrode 210 and the second electrode 220.

Reaction on Negative Electrode

An oxidation reaction occurs on the negative electrode, that is, thesecond electrode 220. Thus, the negative electrode is in a dischargedstate. Thus, electrons are released from the second electrode 220 to theoutside of the flow battery.

For example, in the operation example, the following reaction occurs.

Li→Li⁺ +e ⁻

Reaction on Positive Electrode

When discharging the flow battery, electrons are supplied to thepositive electrode, that is, the first electrode 210 from the outside ofthe flow battery. Thus, a reduction reaction of the first electrodemediator 111 occurs on the first electrode 210. Thus, the firstelectrode mediator 111 is reduced on the surface of the first electrode210.

For example, in the operation example, the following reaction occurs.

TTF²⁺ +e ⁻→TTF⁺

TTF⁺ +e ⁻→TTF

The first circulation mechanism 510 transfers (supplies) the firstelectrode mediator 111 reduced on the first electrode 210 to the firstactive material 310.

The first electrode mediator 111 reduced on the first electrode 210 isoxidized by the first active material 310. In other words, the firstactive material 310 is reduced by the first electrode mediator 111.Thus, the first active material 310 adsorbs lithium.

For example, in the operation example, the following reaction occurs.

FePO₄+Li⁺+TTF→LiFePO₄+TTF⁺

The first circulation mechanism 510 transfers (supplies) the firstelectrode mediator 111 oxidized by the first active material 310 to thefirst electrode 210.

Thus, the first electrode mediator 111 is reduced on the surface of thefirst electrode 210.

For example, in the operation example, the following reaction occurs.

TTF⁺ +e ⁻→TTF

Part of lithium ions (Li⁺) may be supplied from the second electrode 220through the separating unit 400.

Thus, the first electrode mediator 111 is unchanged in the wholereaction including circulation.

Meanwhile, the first active material 310 separated from the firstelectrode 210 is in the discharged state.

Thus, TTF acts as a discharge mediator on the first electrode 210 (afirst electrode side discharge mediator).

In the fully discharged state, the first liquid 110 contains TTF, andthe first active material 310 is LiFePO₄. The discharge potentialdepends on the reduction potential with respect to the direction towardTTF.

The discharge reaction can continue until the first active material 10or the second electrode 220 reaches the fully discharged state.

Second Embodiment

A second embodiment will be described below. The contents described inthe first embodiment are appropriately omitted to avoid overlap.

FIG. 3 is a schematic view of a flow battery 2000 according to a secondembodiment.

In addition to the components of the flow battery 1000 according to thefirst embodiment, the flow battery 2000 according to the secondembodiment further includes the following components.

In the flow battery 2000 according to the second embodiment, the firstcirculation mechanism 510 includes a first container 511.

The first container 511 contains the first active material 310 and thefirst liquid 110.

The first circulation mechanism 510 circulates the first liquid 110between the first electrode 210 and the first container 511.

Contact between the first active material 310 and the first liquid 110in the first container 511 causes at least one of an oxidation reactionand a reduction reaction of the first electrode mediator 111 with thefirst active material 310.

In such a structure, the first liquid 110 can come into contact with thefirst active material 310 in the first container 511. This can increasethe contact area between the first liquid 110 and the first activematerial 310, for example. This can also increase the contact timebetween the first liquid 110 and the first active material 310. This canefficiently promote an oxidation reaction and a reduction reaction ofthe first electrode mediator 111 with the first active material 310.

In the second embodiment, the first container 511 may be a tank.

The first container 511 may contain the first liquid 110, which containsthe first electrode mediator 111 dissolved therein, in voids of thefirst active material 310.

As illustrated in FIG. 3, the flow battery 2000 according to the secondembodiment may further include an electrochemical reaction unit 600, apositive-electrode terminal 211, and a negative-electrode terminal 221.

The electrochemical reaction unit 600 is divided into apositive-electrode chamber 610 and a negative-electrode chamber 620 bythe separating unit 400.

The positive-electrode chamber 610 includes an electrode acting as apositive electrode (the first electrode 210 in FIG. 3).

The positive-electrode terminal 211 is coupled to the electrode actingas a positive electrode.

The negative-electrode chamber 620 includes an electrode acting as anegative electrode (the second electrode 220 in FIG. 3).

The negative-electrode terminal 221 is coupled to the electrode actingas a negative electrode.

The positive-electrode terminal 211 and the negative-electrode terminal221 are coupled to a charge-discharge apparatus, for example. Thecharge-discharge apparatus applies a voltage between thepositive-electrode terminal 211 and the negative-electrode terminal 221or collects electric power generated between the positive-electrodeterminal 211 and the negative-electrode terminal 221.

As illustrated in FIG. 3, in the flow battery 2000 according to thesecond embodiment, the first circulation mechanism 510 may include apipe 514, a pipe 513, and a pump 515.

One end of the pipe 514 is coupled to one of the positive-electrodechamber 610 and the negative-electrode chamber 620 that includes thefirst electrode 210 (the positive-electrode chamber 610 in FIG. 3).

The other end of the pipe 514 is coupled to an inlet of the firstcontainer 511 for the first liquid 110.

One end of the pipe 513 is coupled to an outlet of the first container511 for the first liquid 110.

The other end of the pipe 513 is coupled to one of thepositive-electrode chamber 610 and the negative-electrode chamber 620that includes the first electrode 210 (the positive-electrode chamber610 in FIG. 3).

The pump 515 is disposed on the pipe 514, for example. Alternatively,the pump 515 may be disposed on the pipe 513.

In the flow battery 2000 according to the second embodiment, the firstcirculation mechanism 510 may include a first transfer prevention unit512.

The first transfer prevention unit 512 prevents the transfer of thefirst active material 310.

The first transfer prevention unit 512 is disposed on the path throughwhich the first liquid 110 flows from the first container 511 to thefirst electrode 210 (the pipe 513 in FIG. 3).

Such a structure can prevent the first active material 310 from flowingout of the first container 511 (for example, to the first electrode210). Thus, the first active material 310 remains in the first container511. Thus, the first active material 310 does not circulate in the flowbattery. This can prevent clogging of the first active material 310 in acomponent of the first circulation mechanism 510 (for example, a pipe).This can also prevent resistive loss due to the first active material310 flowing to the first electrode 210.

The first transfer prevention unit 512 may be disposed on the jointbetween the first container 511 and the pipe 513.

For example, the first transfer prevention unit 512 is a filter thatfilters out the first active material 310. The filter may have openingssmaller than the smallest particles of the first active material 310.The filter may be formed of a material that does not react with thefirst active material 310 and the first liquid 110. The filter may be aglass fiber filter paper, polypropylene nonwoven fabric, polyethylenenonwoven fabric, or a metal mesh that does not react with metalliclithium.

Such a structure can prevent the first active material 310 from flowingout of the first container 511 even when the flow of the first liquid110 causes the flow of the first active material 310 in the firstcontainer 511.

In FIG. 3, the first liquid 110 in the first container 511 is suppliedto the positive-electrode chamber 610 through the first transferprevention unit 512 and the pipe 513.

Thus, the first electrode mediator 111 dissolved in the first liquid 110is oxidized or reduced on the first electrode 210.

Subsequently, the first liquid 110 containing the oxidized or reducedfirst electrode mediator 111 dissolved therein is supplied to the firstcontainer 511 through the pipe 514 and the pump 515.

Thus, the first electrode mediator 111 dissolved in the first liquid 110undergoes at least one of an oxidation reaction and a reduction reactionwith the first active material 310.

The circulation of the first liquid 110 may be controlled with the pump515. More specifically, the supply of the first liquid 110 may bestarted or stopped with the pump 515, or the amount of the first liquid110 may be controlled with the pump 515.

Alternatively, the circulation of the first liquid 110 may be controlledby another means (for example, a valve) other than the pump 515.

In FIG. 3, by way of example, the first electrode 210 is a positiveelectrode, and the second electrode 220 is a negative electrode.

If the second electrode 220 has a relatively high electric potential,the first electrode 210 may function as a negative electrode.

Thus, the first electrode 210 may be a negative electrode, and thesecond electrode 220 may be a positive electrode.

Separated by the separating unit 400, the positive-electrode chamber 610and the negative-electrode chamber 620 may contain different electrolytesolutions (solvents).

Alternatively, the positive-electrode chamber 610 and thenegative-electrode chamber 620 may contain the same electrolyte solution(solvent).

Third Embodiment

A third embodiment will be described below. The contents described inthe first or second embodiment are appropriately omitted to avoidoverlap.

In the third embodiment, the electrolyte solution circulates around thefirst electrode and around the second electrode.

FIG. 4 is a block diagram of a flow battery 3000 according to a thirdembodiment;

In addition to the components of the flow battery 1000 according to thefirst embodiment, the flow battery 3000 according to the thirdembodiment further includes the following components.

The flow battery 3000 according to the third embodiment further includesa second liquid 120, the second electrode 220, a second active material320, and the separating unit 400.

The second liquid 120 contains the charge mediator 121 (a secondelectrode side charge mediator) and the discharge mediator 122 (a secondelectrode side discharge mediator) dissolved therein.

The second electrode 220 is immersed in the second liquid 120.

The second active material 320 is immersed in the second liquid 120.

The separating unit 400 separates the first electrode 210 and the firstliquid 110 from the second electrode 220 and the second liquid 120.

The charge mediator 121 has a lower equilibrium potential than thesecond active material 320.

The discharge mediator 122 has a higher equilibrium potential than thesecond active material 320.

Such a structure can provide a flow battery with a higher batteryvoltage, higher energy density, and longer cycle life.

In such a structure, when the second active material 320 is an activematerial with a relatively low equilibrium potential (vs. Li/Li⁺) (forexample, graphite), the discharge mediator 122 may be a substance with arelatively low equilibrium potential (vs. Li/Li⁺) (for example, acondensed aromatic compound). Thus, the negative electrode of the flowbattery can have a lower electric potential. Thus, the flow battery canhave a higher battery voltage (discharge voltage).

Furthermore, such a structure can provide a flow battery in which anactive material is not circulated. Thus, a high-capacity active materialpowder can be used as the second active material 320, for example, in acharge-discharge reaction. Thus, high energy density and capacity can beachieved.

Such a structure can circulate only the second liquid 120 containing thecharge mediator 121 and the discharge mediator 122 dissolved thereinwithout circulating the active material powders. This can reduce theoccurrence of clogging of a pipe with the active material powder. Thus,the flow battery can have a long cycle life.

In the flow battery 3000 according to the third embodiment, the secondliquid 120 may contain lithium dissolved therein.

The second active material 320 may adsorb and desorb lithium.

When charging the flow battery 3000 (electrons are supplied from theoutside of the flow battery 3000 to the second electrode 220), thecharge mediator 121 may be reduced on the second electrode 220, thecharge mediator 121 reduced on the second electrode 220 may be oxidizedby the second active material 320, and the second active material 320may adsorb lithium.

When discharging the flow battery 3000 (electrons are released from thesecond electrode 220 to the outside of the flow battery 3000), thesecond active material 320 on which lithium is adsorbed may reduce thedischarge mediator 122, the second active material 320 may desorblithium, and the discharge mediator 122 reduced by the second activematerial 320 may be oxidized on the second electrode 220.

In such a structure, the second active material 320 may reversiblyadsorb and desorb lithium (for example, lithium ions). This facilitatesthe designing of the second active material 320. This can also furtherincrease the capacity.

In the flow battery 3000 according to the third embodiment, whencharging, the discharge mediator 122 may be reduced on the secondelectrode 220.

When discharging, the charge mediator 121 may be oxidized on the secondelectrode 220.

Such a structure can further increase the energy density and capacity.More specifically, if the discharge mediator 122 is reduced on thesecond electrode 220 when charging, the amount of the discharge mediator122 oxidized on the second electrode 220 when discharging can beincreased. Furthermore, if the charge mediator 121 is oxidized on thesecond electrode 220 when discharging, the amount of the charge mediator121 reduced on the second electrode 220 when charging can be increased.This can increase charge-discharge capacity.

In the flow battery 3000 according to the third embodiment, the chargemediator 121 and the discharge mediator 122 may be condensed aromaticcompounds.

The second liquid 120 containing a condensed aromatic compound dissolvedtherein causes a solvated electron of lithium to be released and therebydissolves the lithium as a cation.

Such a structure can provide the charge mediator 121 and the dischargemediator 122 with a low electric potential. A solution (for example, anether solution) containing a condensed aromatic compound can dissolvelithium (for example, lithium metal). Lithium tends to release anelectron and become a cation. Thus, lithium donates an electron to thecondensed aromatic compound in the solution and dissolves in thesolution as a cation. The condensed aromatic compound that accepts theelectron solvates with the electron. The condensed aromatic compoundsolvated with the electron behaves as an anion. Thus, the solutioncontaining the condensed aromatic compound is ion conductive by itself.The solution containing the condensed aromatic compound contains theequivalent amounts of Li cations and electrons. Thus, the solutioncontaining the condensed aromatic compound can be highly reductive (thatis, have a low electric potential).

For example, an electrode that does not react with lithium immersed inthe second liquid 120 containing a condensed aromatic compound dissolvedtherein has a much lower electric potential than lithium metal. Theelectric potential depends on the degree of solvation between thecondensed aromatic compound and an electron (the type of condensedaromatic compound). Examples of the condensed aromatic compound with alow electric potential include phenanthrene, biphenyl, O-terphenyl,triphenylene, anthracene, phenanthroline, 2,2′-bipyridyl, benzophenone,trans-stilbene, 4,4′-bipyridyl, 3,3′-bipyridyl, 2,4′-bipyridyl,2,3′-bipyridyl, cis-stilbene, acetophenone, propiophenone,butyrophenone, valerophenone, and ethylenediamine.

In the flow battery 3000 according to the third embodiment, the chargemediator 121 may be at least one selected from the group consisting ofphenanthrene, biphenyl, O-terphenyl, triphenylene, and anthracene.

Such a structure can provide the charge mediator 121 with a low electricpotential. More specifically, a charge mediator with a lower electricpotential (vs. Li/Li⁺) than the second active material 320 (for example,graphite) can be obtained.

In the flow battery 3000 according to the third embodiment, thedischarge mediator 122 may be at least one selected from the groupconsisting of phenanthroline, 2,2′-bipyridyl; benzophenone,trans-stilbene, 4,4′-bipyridyl, 3,3′-bipyridyl, 2,4′-bipyridyl,2,3′-bipyridyl, cis-stilbene, acetophenone, propiophenone,butyrophenone, valerophenone, and ethylenediamine.

Such a structure can provide the discharge mediator 122 with a highelectric potential. More specifically, the discharge mediator 122 with ahigher electric potential (vs. Li/Li⁺) than the second active material320 (for example, graphite) can be obtained.

In the flow battery 3000 according to the third embodiment, thedischarge mediator 122 may be at least one selected from the groupconsisting of 2,2′-bipyridyl, stilbene, 2,4′-bipyridyl, 2,3′-bipyridyl,cis-stilbene, propiophenone, butyrophenone, valerophenone, andethylenediamine.

Such a structure can decrease the equilibrium potential (vs. Li/Li⁺) ofthe discharge mediator 122. Thus, the negative electrode of the flowbattery can have a lower electric potential. Thus, the flow battery canhave a higher battery voltage (discharge voltage).

In the flow battery 3000 according to the third embodiment, the secondliquid 120 may be an ether solution.

In such a structure, the second liquid 120 can be an electrolytesolution containing the charge mediator 121 and the discharge mediator122. More specifically, the solvent of the charge mediator 121 and thedischarge mediator 122 can be an electronically non-conductive ethersolution, and the ether solution itself can have the properties of anelectrolyte solution.

The ether may be tetrahydrofuran (THF), 2-methyltetrahydrofuran(2MeTHF), dimethoxyethane (DME), 1,3-dioxane (1,3DO), or4-methyl-1,3-dioxane (4Me1,3DO).

In the flow battery 3000 according to the third embodiment, the secondactive material 320 may be graphite.

Such a structure can decrease the equilibrium potential (vs. Li/Li⁺) ofthe second active material 320. Thus, the discharge mediator 122 can bea substance with a relatively low equilibrium potential (vs. Li/Li⁺)(for example, a condensed aromatic compound). Thus, the negativeelectrode of the flow battery can have a lower electric potential. Thus,the flow battery can have a high battery voltage (discharge voltage).

In the third embodiment, graphite of the second active material 320 onwhich lithium is adsorbed (a graphite interlayer compound produced whencharging) may have a composition of at least one of C₂₄Li, C₁₆Li, C₁₂Li,and C₆Li.

When the second active material 320 is graphite (C₆Li), charginginvolves complete reduction by lithium (graphite adsorbs lithium toyield C₆Li). C₆Li has an electric potential of approximately 0.2 V vs.Li/Li⁺. Thus, a mediator-type negative electrode can be formed by usinga condensed aromatic compound with a lower electric potential than C₆Lias a charge mediator and a condensed aromatic compound with a higherelectric potential than C₆Li as a discharge mediator.

FIG. 5 is a table that lists the electric potentials of condensedaromatic compounds usable as a charge mediator 121.

FIG. 6 is a table that lists the electric potentials of condensedaromatic compounds usable as a discharge mediator 122.

A 2×2 cm copper foil is covered with a polypropylene microporousseparator, which is covered with a large amount of lithium metal foil. Atab is attached to the copper foil and lithium metal. Subsequently, alaminate exterior is provided. After pouring 2MeTHF in which a condensedaromatic compound is dissolved at a molar concentration (M) listed inFIGS. 5 and 6, the laminate exterior is hermetically sealed by heat.Thus, an electrometric cell for each condensed aromatic compound isprepared. FIGS. 5 and 6 list the electric potentials (V vs. Li/Li⁺)based on lithium metal measured with the electrometric cells. Although2MeTHF can be used in this measurement, another ether may also be used.

The charge mediator 121 cannot dissolve Li of C₆Li. By contrast, thedischarge mediator 122 can dissolve Li of C₆Li. This difference resultsfrom the difference between the electric potential of C₆Li and theelectric potentials of these lithium metal solutions. Those with ahigher electric potential than C₆Li (approximately 0.2 V vs. Li/Li⁺) candissolve Li of C₆Li. By contrast, those with a lower electric potentialthan C₆Li (approximately 0.2 V vs. Li/Li⁺) cannot dissolve Li of C₆Li.

Thus, those with a lower electric potential than C₆Li can be used as thecharge mediator 121. Those with a higher electric potential than C₆Lican be used as the discharge mediator 122.

A smaller potential difference between the condensed aromatic compoundand the second active material 320 results in higher charge-dischargeenergy efficiency. When the second active material 320 is graphite(C₆Li), therefore, the charge mediator 121 may be phenanthrene,triphenylene, or biphenyl. The discharge mediator 122 may betrans-stilbene, butyrophenone, valerophenone, or ethylenediamine. Thiscan further increase charge-discharge energy efficiency.

Unlike Li ions, the ether may not be intercalated into graphite. Noco-intercalation of Li and the ether in graphite can increase capacitydensity.

The second active material 320 may be a solid active material (forexample, a powdered active material). If the second active material 320is stored as an unprocessed powder in a tank, this can simplify theproduction and reduce production costs.

Alternatively, the second active material 320 may be a pelleted activematerial (for example, a powder is pelleted). If the second activematerial 320 is stored as pellets in a tank, this can simplify theproduction and reduce production costs.

The second active material 320 may be pelleted with a generally knownbinder (for example, poly(vinylidene difluoride), polypropylene,polyethylene, or polyimide).

The second active material 320 may be insoluble in the second liquid120. Thus, there is provided a flow battery in which the charge mediator121 and the discharge mediator 122 as well as the second liquid 120circulate, but the second active material 320 does not circulate.

In the flow battery 3000 according to the third embodiment, the secondelectrode 220 may be a negative electrode, and the first electrode 210may be a positive electrode.

If the first electrode 210 has a relatively low electric potential, thesecond electrode 220 may function as a positive electrode.

Thus, the second electrode 220 may be a positive electrode, and thefirst electrode 210 may be a negative electrode.

In the flow battery 3000 according to the third embodiment, for example,when the second liquid 120 comes into contact with the second electrode220, the charge mediator 121 and the discharge mediator 122 are oxidizedor reduced on the second electrode 220. For example, when the secondliquid 120 comes into contact with the second active material 320, thesecond active material 320 causes a reduction reaction of the dischargemediator 122 or an oxidation reaction of the charge mediator 121.

The second electrode 220 may have a surface that acts as a reactionfield for the charge mediator 121 and the discharge mediator 122.

In this case, the material of the second electrode 220 may be stable inthe second liquid 120 (for example, a material insoluble in the secondliquid 120). The material of the second electrode 220 may also beresistant to an electrode reaction, that is, an electrochemicalreaction. For example, the second electrode 220 may be formed of a metal(stainless steel, iron, copper, or nickel) or carbon.

The second electrode 220 may have a structure with an increased surfacearea (for example, a mesh, nonwoven fabric, surface roughened plate, orsintered porous body). Thus, the second electrode 220 may have a largespecific surface area. This can promote an oxidation or reductionreaction of the charge mediator 121 and the discharge mediator 122.

The flow battery 3000 according to the third embodiment may furtherinclude a second circulation mechanism 520.

The second circulation mechanism 520 circulates the second liquid 120between the second electrode 220 and the second active material 320.

Such a structure can circulate the charge mediator 121 and the dischargemediator 122 together with the second liquid 120 between the secondelectrode 220 and the second active material 320. This can efficientlypromote an oxidation reaction and a reduction reaction betweenmaterials.

The second circulation mechanism 520 may include a pipe, a tank, a pump,and a valve, for example.

A specific example of the second circulation mechanism 520 may be astructure described later in a fourth embodiment.

<Charge-Discharge Process>

The charge-discharge process of the flow battery 3000 according to thethird embodiment will be described below.

The charge-discharge process is specifically described with thefollowing operation example.

In the operation example, the first electrode 210 is a positiveelectrode made of carbon black.

In the operation example, the first liquid 110 is an ether solutioncontaining the first electrode mediator 111 dissolved therein.

In the operation example, the first electrode mediator 111 is atetrathiafulvalene derivative according to the first embodiment(hereinafter referred to as “TTF”).

In the operation example, the first active material 310 is lithium ironphosphate (LiFePO₄).

In the operation example, the second electrode 220 is a negativeelectrode made of stainless steel.

In the operation example, the second liquid 120 is an ether solutioncontaining the charge mediator 121 and the discharge mediator 122dissolved therein.

In the operation example, the charge mediator 121 on the side of thesecond electrode 220 is a condensed aromatic compound (hereinafterreferred to as ChMd).

In the operation example, the discharge mediator 122 on the side of thesecond electrode 220 is a condensed aromatic compound (hereinafterreferred to as DchMd).

In the operation example, the second active material 320 is graphite(C₆Li).

In the operation example, the separating unit 400 is a lithium ionconductive solid electrolyte membrane.

[Charging Process]

First, a charge reaction will be described below.

A voltage is applied between the first electrode 210 and the secondelectrode 220 when charging.

Reaction on Negative Electrode

Upon application of a voltage, electrons are supplied to the negativeelectrode, that is, the second electrode 220 from the outside of theflow battery. This causes a reduction reaction of the charge mediator121 and the discharge mediator 122 on the second electrode 220.

For example, in the operation example, the following reaction occurs.

ChMd+Li⁺ +e ⁻→ChMd.Li

DchMd+Li⁺ +e ⁻→DchMd.Li

The second circulation mechanism 520 transfers (supplies) the chargemediator 121 reduced on the second electrode 220 to the second activematerial 320.

The charge mediator 121 reduced on the second electrode 220 is oxidizedby the second active material 320. In other words, the second activematerial 320 is reduced by the charge mediator 121. Thus, the secondactive material 320 adsorbs lithium and becomes C₆Li.

For example, in the operation example, the following reaction occurs.

6C+ChMd.Li→C₆Li+ChMd

The second circulation mechanism 520 transfers (supplies) the chargemediator 121 oxidized by the second active material 320 to the secondelectrode 220.

Thus, the charge mediator 121 is unchanged in the whole reactionincluding circulation.

Meanwhile, the second active material 320 separated from the secondelectrode 220 is charged.

Reaction on Positive Electrode

Upon application of a voltage, an oxidation reaction of the firstelectrode mediator 111 occurs on the positive electrode, that is, thefirst electrode 210. Thus, the first electrode mediator 111 is oxidizedon the surface of the first electrode 210. Thus, electrons are releasedfrom the first electrode 210 to the outside of the flow battery.

For example, in the operation example, the following reaction occurs.

TTF→TTF⁺ +e ⁻

TTF⁺→TTF²⁺ +e ⁻

The first circulation mechanism 510 transfers (supplies) the firstelectrode mediator 111 oxidized on the first electrode 210 to the firstactive material 310.

The first electrode mediator 111 oxidized on the first electrode 210 isreduced by the first active material 310. In other words, the firstactive material 310 is oxidized by the first electrode mediator 111.Thus, the first active material 310 desorbs lithium.

For example, in the operation example, the following reaction occurs.

LiFePO₄+TTF²⁺→FePO₄+Li⁺+TTF⁺

The first circulation mechanism 510 transfers (supplies) the firstelectrode mediator 111 reduced by the first active material 310 to thefirst electrode 210.

Thus, the first electrode mediator 111 is oxidized on the surface of thefirst electrode 210.

For example, in the operation example, the following reaction occurs.

TTF⁺→TTF²⁺ +e ⁻

Part of lithium ions (Li⁺) thus desorbed may move to the secondelectrode 220 through the separating unit 400.

Thus, the first electrode mediator 111 is unchanged in the wholereaction including circulation.

Meanwhile, the first active material 310 separated from the firstelectrode 210 is charged.

Thus, TTF⁺ acts as a charge mediator on the first electrode 210 (a firstelectrode side charge mediator).

In the fully charged state, the first liquid 110 contains TTF²⁺, and thefirst active material 310 is FePO₄. The charge potential depends on theoxidation potential with respect to the direction toward TTF²⁺.

The charge reaction can continue until the first active material 310 orthe second active material 320 reaches the fully charged state.

[Discharge Process]

A discharge reaction starting from the fully charged state will bedescribed below.

In the fully charged state, the first active material 310 and the secondactive material 320 are in the charged state.

During the discharge reaction, electric power is generated between thefirst electrode 210 and the second electrode 220.

Reaction on Negative Electrode

Battery discharge causes an oxidation reaction of the charge mediator121 and the discharge mediator 122 on the negative electrode, that is,the second electrode 220. Thus, electrons are released from the secondelectrode 220 to the outside of the flow battery.

For example, in the operation example, the following reaction occurs.

DchMd.Li→DchMd+Li⁺ +e ⁻

ChMd.Li→ChMd+Li⁺ +e ⁻

The second circulation mechanism 520 transfers (supplies) the dischargemediator 122 oxidized on the second electrode 220 to the second activematerial 320.

The discharge mediator 122 oxidized on the second electrode 220 isreduced by the second active material 320. In other words, the secondactive material 320 is oxidized by the discharge mediator 122. Thus, thesecond active material 320 desorbs lithium.

For example, in the operation example, the following reaction occurs.

C₆Li+DchMd→6C+DchMd.Li

The second circulation mechanism 520 transfers (supplies) the dischargemediator 122 reduced by the second active material 320 to the secondelectrode 220.

Thus, the discharge mediator 122 is unchanged in the whole reactionincluding circulation.

Meanwhile, the second active material 320 separated from the secondelectrode 220 is in the discharged state.

Reaction on Positive Electrode

When discharging the flow battery, electrons are supplied to thepositive electrode, that is, the first electrode 210 from the outside ofthe flow battery. Thus, a reduction reaction of the first electrodemediator 111 occurs on the first electrode 210. Thus, the firstelectrode mediator 111 is reduced on the surface of the first electrode210.

For example, in the operation example, the following reaction occurs.

TTF²⁺ +e ⁻→TTF⁺

TTF⁺ +e ⁻→TTF

The first circulation mechanism 510 transfers (supplies) the firstelectrode mediator 111 reduced on the first electrode 210 to the firstactive material 310.

The first electrode mediator 111 reduced on the first electrode 210 isoxidized by the first active material 310. In other words, the firstactive material 310 is reduced by the first electrode mediator 111.Thus, the first active material 310 adsorbs lithium.

For example, in the operation example, the following reaction occurs.

FePO₄+Li⁺+TTF→LiFePO₄+TTF⁺

The first circulation mechanism 510 transfers (supplies) the firstelectrode mediator 111 oxidized by the first active material 310 to thefirst electrode 210.

Thus, the first electrode mediator 111 is reduced on the surface of thefirst electrode 210.

For example, in the operation example, the following reaction occurs.

TTF⁺ +e ⁻→TTF

Part of lithium ions (Li⁺) may be supplied from the second electrode 220through the separating unit 400.

Thus, the first electrode mediator 111 is unchanged in the wholereaction including circulation.

Meanwhile, the first active material 310 separated from the firstelectrode 210 is in the discharged state.

Thus, TTF acts as a discharge mediator on the first electrode 210 (afirst electrode side discharge mediator).

In the fully discharged state, the first liquid 110 contains TTF, andthe first active material 310 is LiFePO₄. The discharge potentialdepends on the reduction potential with respect to the direction towardTTF.

The discharge reaction can continue until the first active material 310or the second active material 320 reaches the fully discharged state.

<Estimation of Energy Density>

The estimated energy density of the flow battery 3000 according to thethird embodiment will be described below.

FIG. 7 is a table that lists the estimated energy density of the flowbattery 3000 according to the third embodiment.

FIG. 7 lists the estimated energy densities under the conditions of theoperation example of the flow battery 3000 according to the thirdembodiment. Each of the compounds listed in FIG. 2 is used as the firstelectrode mediator 111. The charge mediator 121 is phenanthrene, and thedischarge mediator 122 is trans-stilbene.

As shown in FIG. 7, the flow battery can have an energy density ofapproximately 642 to 652 Wh/L when a tetrathiafulvalene derivative witha chain-form ing substituent (electron-withdrawing substituent) atpositions 4,4′ and 5,5′ of the tetrathiafulvalene skeleton thereof isused as the first electrode mediator 111.

By contrast, the theoretical energy densities of known flow batteries(utilizing vanadium) are approximately 38 Wh/L. Thus, the flow batteriesaccording to the present disclosure have significantly highertheoretical energy densities than known flow batteries.

As shown in FIG. 7, when a tetrathiafulvalene with no chain-formingsubstituent is used as the first electrode mediator 111, the flowbattery has an energy density of approximately 633 Wh/L. The resultsshow that the flow batteries containing a tetrathiafulvalene derivativewith a chain-forming substituent have a higher theoretical energydensity than those containing a tetrathiafulvalene with no chain-formingsubstituent.

Fourth Embodiment

A fourth embodiment will be described below. The contents described inthe first to third embodiments are appropriately omitted to avoidoverlap.

FIG. 8 is a schematic view of a flow battery 4000 according to a fourthembodiment.

In addition to the components of the flow battery 3000 according to thethird embodiment, the flow battery 4000 according to the fourthembodiment further includes the following components.

First, the flow battery 4000 according to the fourth embodiment includesthe first circulation mechanism 510 described in the second embodiment.

The flow battery 4000 according to the fourth embodiment furtherincludes the electrochemical reaction unit 600, the positive-electrodeterminal 211, and the negative-electrode terminal 221 described in thesecond embodiment.

The flow battery 4000 according to the fourth embodiment furtherincludes the second circulation mechanism 520.

The second circulation mechanism 520 includes a second container 521.

The second container 521 contains the second active material 320 and thesecond liquid 120.

The second circulation mechanism 520 circulates the second liquid 120between the second electrode 220 and the second container 521.

In the second container 521, contact between the second active material320 and the second liquid 120 causes at least one of an oxidationreaction of the charge mediator 121 with the second active material 320and a reduction reaction of the discharge mediator 122 with the secondactive material 320.

In such a structure, the second liquid 120 can come into contact withthe second active material 320 in the second container 521. This canincrease the contact area between the second liquid 120 and the secondactive material 320, for example. This can also increase the contacttime between the second liquid 120 and the second active material 320.This can efficiently promote an oxidation reaction of the chargemediator 121 with the second active material 320 and a reductionreaction of the discharge mediator 122 with the second active material320.

In the fourth embodiment, the second container 521 may be a tank.

The second container 521 may contain the second liquid 120, whichcontains the charge mediator 121 and the discharge mediator 122dissolved therein, in voids of the second active material 320.

As illustrated in FIG. 8, in the flow battery 4000 according to thefourth embodiment, the second circulation mechanism 520 may include apipe 523, a pipe 524, and a pump 525.

One end of the pipe 524 is coupled to one of the positive-electrodechamber 610 and the negative-electrode chamber 620 that includes thesecond electrode 220 (the negative-electrode chamber 620 in FIG. 8).

The other end of the pipe 524 is coupled to an inlet of the secondcontainer 521 for the second liquid 120.

One end of the pipe 523 is coupled to an outlet of the second container521 for the second liquid 120.

The other end of the pipe 523 is coupled to one of thepositive-electrode chamber 610 and the negative-electrode chamber 620that includes the second electrode 220 (the negative-electrode chamber620 in FIG. 8).

The pump 525 is disposed on the pipe 524, for example. Alternatively,the pump 525 may be disposed on the pipe 523.

In the flow battery 4000 according to the fourth embodiment, the secondcirculation mechanism 520 may include a second transfer prevention unit522.

The second transfer prevention unit 522 prevents the transfer of thesecond active material 320.

The second transfer prevention unit 522 is disposed on the path throughwhich the second liquid 120 flows from the second container 521 to thesecond electrode 220 (the pipe 523 in FIG. 8).

Such a structure can prevent the second active material 320 from flowingout of the second container 521 (for example, to the second electrode220). Thus, the second active material 320 remains in the secondcontainer 521. Thus, the second active material 320 does not circulatein the flow battery. This can prevent clogging of the second activematerial 320 in a component of the second circulation mechanism 520 (forexample, a pipe). This can also prevent resistive loss due to the secondactive material 320 flowing to the second electrode 220.

The second transfer prevention unit 522 may be disposed on the jointbetween the second container 521 and the pipe 523.

For example, the second transfer prevention unit 522 is a filter thatfilters out the second active material 320. The filter may have openingssmaller than the smallest particles of the second active material 320.The filter may be formed of a material that does not react with thesecond active material 320 and the second liquid 120. The filter may bea glass fiber filter paper, polypropylene nonwoven fabric, polyethylenenonwoven fabric, or a metal mesh that does not react with metalliclithium.

Such a structure can prevent the second active material 320 from flowingout of the second container 521 even when the flow of the second liquid120 causes the flow of the second active material 320 in the secondcontainer 521.

In FIG. 8, the second liquid 120 in the second container 521 is suppliedto the negative-electrode chamber 620 through the second transferprevention unit 522 and the pipe 523.

Thus, the charge mediator 121 and the discharge mediator 122 dissolvedin the second liquid 120 is oxidized or reduced on the second electrode220.

Subsequently, the second liquid 120 containing the oxidized or reducedcharge mediator 121 and discharge mediator 122 dissolved therein issupplied to the second container 521 through the pipe 524 and the pump525.

Thus, the charge mediator 121 and the discharge mediator 122 dissolvedin the second liquid 120 undergo at least one of an oxidation reactionof the charge mediator 121 with the second active material 320 and areduction reaction of the discharge mediator 122 with the second activematerial 320.

The circulation of the second liquid 120 may be controlled with the pump525. More specifically, the supply of the second liquid 120 may bestarted or stopped with the pump 525, or the amount of the second liquid120 may be controlled with the pump 525.

Alternatively, the circulation of the second liquid 120 may becontrolled by another means (for example, a valve) other than the pump525.

In FIG. 8, by way of example, the first electrode 210 is a positiveelectrode, and the second electrode 220 is a negative electrode.

If the first electrode 210 has a relatively low electric potential, thesecond electrode 220 may function as a positive electrode.

Thus, the second electrode 220 may be a positive electrode, and thefirst electrode 210 may be a negative electrode.

Tetrathiafulvalene derivatives according to the present disclosure(tetrathiafulvalene derivatives with a chain-forming substituent atpositions 4,4′ and 5,5′ of the tetrathiafulvalene skeleton) can havehigher solubility in the first liquid 110 than tetrathiafulvalenederivatives with a ring-forming substituent at positions 4,4′ and 5,5′of the tetrathiafulvalene skeleton.

The constituents of the first to fourth embodiments may be appropriatelycombined.

A flow battery according to the present disclosure can be suitable forcharge storage devices and charge storage systems, for example.

What is claimed is:
 1. A flow battery comprising: a first liquidcontaining a first electrode mediator dissolved therein; a firstelectrode immersed in the first liquid; a first active material immersedin the first liquid; and a first circulation mechanism that circulatesthe first liquid between the first electrode and the first activematerial, wherein the first electrode mediator includes atetrathiafulvalene derivative, and the tetrathiafulvalene derivative hasa chain-forming substituent at positions 4,4′ and 5,5′ of atetrathiafulvalene skeleton thereof.
 2. The flow battery according toclaim 1, wherein the tetrathiafulvalene derivative is represented by thefollowing general formula (1),

wherein X denotes an oxygen atom, a sulfur atom, a nitrogen atom, aselenium atom, or a tellurium atom, and R₁, R₂, R₃, and R₄ independentlydenote at least one selected from the group consisting of chainsaturated hydrocarbons, chain unsaturated hydrocarbons, cyclic saturatedhydrocarbons; cyclic unsaturated hydrocarbons, a phenyl group, ahydrogen atom, a hydroxy group, a cyano group, an amino group, a nitrogroup; and a nitroso group.
 3. The flow battery according to claim 2,wherein at least one electron-withdrawing group selected from the groupconsisting of a sulfur atom, a nitrogen atom, and an oxygen atom isdisposed at position X.
 4. The flow battery according to claim 2,wherein a linear substituent is disposed at positions R₁, R₂, R₃, andR₄.
 5. The flow battery according to claim 2, wherein a substituent atpositions R₁, R₂, R₃, and R₄ is C_(n)H_(2n+1) (n is an integer of1≦n≦4).
 6. The flow battery according to claim 2, wherein a substituentat positions R₁, R₂, R₃, and R₄ is a branched substituent.
 7. The flowbattery according to claim 2, wherein a substituent at positions R₁, R₂,R₃, and R₄ includes at least one element selected from the groupconsisting of boron, nitrogen, oxygen, fluorine, silicon, phosphorus,sulfur, chlorine, bromine, and iodine.
 8. The flow battery according toclaim 2, wherein the tetrathiafulvalene derivative is at least oneselected from the group consisting oftetrakis(dimethylthio)tetrathiafulvalene andtetrakis(diethylthio)tetrathiafulvalene.
 9. The flow battery accordingto claim 1, wherein the tetrathiafulvalene derivative has a firstoxidation-reduction potential and a second oxidation-reductionpotential, and the first active material has an equilibrium potentialhigher than the first oxidation-reduction potential and lower than thesecond oxidation-reduction potential.
 10. The flow battery according toclaim 1, wherein the first circulation mechanism includes a firstcontainer, the first container contains the first active material andthe first liquid, the first circulation mechanism circulates the firstliquid between the first electrode and the first container, and contactbetween the first active material and the first liquid in the firstcontainer causes at least one of an oxidation reaction and a reductionreaction between the first active material and the first electrodemediator.
 11. The flow battery according to claim 10, wherein the firstcirculation mechanism includes a first transfer prevention unit thatprevents transfer of the first active material, and the first transferprevention unit is disposed on a path through which the first liquidflows from the first container to the first electrode.
 12. The flowbattery according to claim 1, further comprising: a second liquidcontaining a charge mediator and a discharge mediator dissolved therein;a second electrode immersed in the second liquid; a second activematerial immersed in the second liquid; and a separating unit thatseparates the first electrode and the first liquid from the secondelectrode and the second liquid, wherein the charge mediator has a lowerequilibrium potential than the second active material, and the dischargemediator has a higher equilibrium potential than the second activematerial.
 13. The flow battery according to claim 12, wherein the secondliquid contains lithium dissolved therein, the second active materialadsorbs and desorbs the lithium, when charging, the charge mediator isreduced on the second electrode, the charge mediator reduced on thesecond electrode is oxidized by the second active material, and thesecond active material adsorbs the lithium, and when discharging, thesecond active material on which the lithium is adsorbed reduces thedischarge mediator, the second active material desorbs the lithium, andthe discharge mediator reduced by the second active material is oxidizedon the second electrode.
 14. The flow battery according to claim 13,wherein when the charging, the discharge mediator is reduced on thesecond electrode, and when the discharging, the charge mediator isoxidized on the second electrode.
 15. The flow battery according toclaim 12, wherein the charge mediator and the discharge mediator arecondensed aromatic compounds, and the second liquid containing thecondensed aromatic compounds dissolved therein causes a solvatedelectron of lithium to be released and thereby dissolves the lithium asa cation.
 16. The flow battery according to claim 15, wherein the chargemediator is at least one selected from the group consisting ofphenanthrene, biphenyl, O-terphenyl, triphenylene, and anthracene. 17.The flow battery according to claim 15, wherein the discharge mediatoris at least one selected from the group consisting of phenanthroline,2,2′-bipyridyl, benzophenone, trans-stilbene, 4,4′-bipyridyl,3,3′-bipyridyl, 2,4′-bipyridyl, 2,3′-bipyridyl, cis-stilbene,acetophenone, propiophenone, butyrophenone, valerophenone, andethylenediamine.
 18. The flow battery according to claim 12, furthercomprising: a second circulation mechanism including a second container,wherein the second active material and the second liquid are containedin the second container, the second circulation mechanism circulates thesecond liquid between the second electrode and the second container, andcontact between the second active material and the second liquid in thesecond container causes at least one of an oxidation reaction of thecharge mediator with the second active material and a reduction reactionof the discharge mediator with the second active material.
 19. The flowbattery according to claim 18, wherein the second circulation mechanismincludes a second transfer prevention unit that prevents transfer of thesecond active material, and the second transfer prevention unit isdisposed on a path through which the second liquid flows from the secondcontainer to the second electrode.